MONDAY, May 27 (HealthDay News) -- Legalizing marijuana may have unintended consequences. Since medical marijuana was legalized in Colorado, more than a dozen young children have been unintentionally poisoned with the drug, researchers report.

About half the cases resulted from kids eating marijuana-laced cookies, brownies, sodas or candy. In many cases, the marijuana came from their grandparents' stash, the investigators said.

"We are seeing increases in exposure to marijuana in young pediatric patients, and they have more severe symptoms than we typically associate with marijuana," said lead researcher Dr. George Sam Wang, a medical toxicology fellow at the Rocky Mountain Poison and Drug Center in Denver.

But doctors aren't familiar with marijuana poisoning in children, so unless the parents are forthcoming it can take time and tests to diagnose the problem, Wang said. Symptoms of marijuana poisoning in children include sleepiness and balance problems while walking.

"We hadn't seen these exposures before the big boom of the medical marijuana industry," Wang said.

The active chemical in marijuana, tetrahydrocannabinol, is in higher than normal concentrations in medical marijuana, and often is sold in baked goods, soft drinks and candies, the researchers said in the study, which was published online May 27 in the journal JAMA Pediatrics.

"We are seeing more symptoms because some of these products have very high amounts of marijuana in them," Wang said. "You get such a high dose on such a small child, the symptoms are more severe."

As with many similar poisonings, treatment is limited to supportive care and waiting until the marijuana clears the system, he said.

Children recover quickly in most cases, Wang said. "They don't need more than a day or two of hospitalization," he said. "There were no deaths or lasting side effects."

This report stems from one Denver hospital, and Wang said he doesn't know how extensive the problem is elsewhere. Colorado adults are allowed to possess up to 1 ounce of marijuana or six marijuana plants, according to the study. And Denver issued more than 300 sales tax licenses for marijuana dispensaries in 2010.

For the study, Wang's team compared the number of children treated in the emergency room for marijuana poisoning before and after the law was enacted in October 2009.

In all, almost 1,400 children under 12 were evaluated for accidental poisonings in this one hospital -- 790 before Sept. 30, 2009, and 588 after that.

After decriminalization, 14 children -- mostly boys and some as young as 8 months -- were found to have ingested marijuana. Eight had consumed medical marijuana, and seven ate marijuana in foods. Two were admitted to the intensive care unit.

Before Sept. 30, 2009, none of those possible poisonings was attributed to marijuana, the researchers found.

There may be more unreported cases, the study authors said. "Because of a perceived stigma associated with medical marijuana, families may be reluctant to report its use to health care providers," they wrote in the study.

"Similar to many accidental medicinal pediatric exposures, the source of the marijuana in most cases was the grandparents, who may not have been available during data collection," the researchers added.

Eighteen states and Washington, D.C., have legalized medical marijuana. Colorado and Washington also have legalized the recreational use of marijuana.

In late 2009, the U.S. Justice Department instructed federal prosecutors not to arrest medical marijuana users and suppliers if they complied with state laws, the researchers said.

To prevent harm to children, Wang advises treating marijuana like any other drug and keeping it out of their reach, particularly if it's in a tempting form like cookies.

Some poison-control experts also are pushing for marijuana to come in tamper-proof packages as a way of keeping children away from it.

The ongoing debate about legalizing marijuana should include discussion of the potential consequences to children, said the researchers and other medical experts.

"There is a lot of information that may not be entirely accurate about how benign marijuana is," said Dr. Sharon Levy, an assistant professor of pediatrics at Harvard Medical School, who wrote an accompanying journal editorial.